A superconducting coil has been put to practical use in various fields as a means of generating high magnetic fields. On the other hand, the practical application of superconducting coils to AC devices, such as transformers and reactors, has made little progress due to the phenomenon of losses incurred by superconducting conductors in the presence of AC. However, since the recent development of a superconducting conductor having a small loss of AC by the thinning of superconducting stranded wires, a progress has been made in the researches for its application to transformers and other AC devices, and various proposals have been made on the structure of superconducting coils made thereof.
As superconducting conductors for this case, a superconducting wire made of a metal superconductor that remains in a superconducting state at a very low temperature of 4K at which liquid helium evaporates is mainly used as a practical superconducting material. Recently, however, efforts have been made to develop superconducting coils based on an oxide superconductor. This oxide superconductor is also called “a high-temperature superconductor.” This high temperature superconductor is more advantageous than metallic superconductors in terms of a lower operating cost.
When a plurality of conductors are used in parallel in an AC equipment, such as a transformer in which current varies at a high speed, conductors are transposed. The relative positions of a plurality of conductors are changed to reduce the interlinkage magnetic flux between the respective conductors, or to reduce induced voltage resulting therefrom, to thereby make the current distribution for the respective conductors uniform. The differences in induced voltage between respective parallel conductors resulting from the magnetic flux generated by current induces circulating current. In the case of ordinary or non-superconducting conductors, such as copper or aluminum, however, impedance consists mainly of resistance component and the circulating current has a phase deviating by approximately 90° in relation to the load current. For this reason, even if a 30% circulating current is generated, the current flowing in a conductor is the vector sum of 100% of the load current and a 30% circulating current having a phase difference of 90° thereto, and therefore, the absolute value thereof which is the square root of the sum of respective squares amounts to approximately 105%. Thus, the increase in the value of current is small for the circulating current.
When a superconducting wire is used as a conductor, on the other hand, as resistance is practically zero in the superconducting state, impedance that determines circulating current is mostly determined by inductance. Therefore, the circulating current takes the same phase as current, and if the circulating current is 30%, this circulating current is added to the current and as a result a 130% current flows in the superconductor. When this current value reaches the critical current level, however, the loss of AC increases or drift increases.
There exists a critical temperature, a critical current or a critical magnetic field on the superconducting conductor (or superconducting wire) used in the winding of a superconducting coil. In other words, To enable the superconducting wire to maintain the superconducting state, it is necessary to keep the temperature, current, and magnetic field below the specific critical values. When current above the critical current flows in the superconducting wire due to the circulating current, the superconducting wire shifts from the superconducting state to the normal conducting state. In other words, it turns into a normal conductor having resistance. Moreover, the superconducting wire can be damaged by the Joule heat generation. Thus, it is very important to suppress circulating current in a coil consisting of a superconducting wire. For this purpose, transposition is carried out and circulating current is controlled as mentioned earlier. Moreover, the oxide superconducting wire is more vulnerable to bending force than alloy superconductors, and there is an allowable bending radius for displaying its capacity. Therefore, the number of instable points increases as the number of superconductors arranged in parallel increases, in other words as the number of transposed parts increases. Thus, a meticulous care is needed in any transposition work.
The structure of a superconducting coil designed to simplify transposing work and lower costs by reducing transposition parts serving as instable points and suppressing circulating current is disclosed, for example, in Japanese Patent Application Laid Open 11-273935 (pp. 2-4, FIGS. 1-4) (hereafter Reference 1). The summary of the invention described in Reference 1 is as follows: “[I]n a superconducting coil in which a plurality of superconducting wires are arranged in parallel and wound, it is possible to reduce the number of transposition parts, contain the circulating current and at the same time reduce the unstable parts by adopting a structure in which the relative positions are changed only at the ends of coil, and in addition by making the number of coil layers an integral multiple of 4 times the number of superconducting wires arranged in parallel (4 times the number of wires). As a result, the work and time for transposition is reduced resulting not only in lower costs, but also fewer unstable parts and thus enabling to contain circulating current. Therefore, it is possible to obtain an advantage of being able to excite and demagnetize at a high speed and stably”.
FIG. 7 is an example of the transposition structure of a superconducting coil described in FIG. 1 of Reference 1. In FIG. 7, for winding three superconducting wires 3a superposed in the radial direction of the coil by winding in the direction of bobbin 1a-bobbin 1b, at the start of the coil on the 1a side of the bobbin, the superconducting wires 3a are wound for multiple layers and from the internal diameter of the coil, for example, in the order of A1, A2, and A3 (not shown), and at the transposition part 2b at the end of the coil, at first A3 is bent at the following turn, and the transposition work is carried out on A2 and A1 in the same manner, so that at the end of the coil on the 1b side of the bobbin, the coil will be arranged for example in the order of A3, A2, and A1. By arranging the same as described above, the number of transposition parts and bending of coil will be reduced in comparison with the prior transposition structure described in FIG. 4 of Reference 1, and the work will be considerably simplified thereby. Regarding an example of the structure mentioned above on a number of coil layers equal to an integral multiple of four times the number of superconducting wires arranged in parallel (4 times the number of wires), the description is omitted here. See Reference 1 for details.
The adoption of a transposition structure as described in Reference 1 will enable the inductance and current distribution for the respective superconducting wires constituting the conductor to be uniform. This will increase the current capacity by increasing the number of superconducting wires arranged in parallel and to eliminate additional losses due to the increased number of superconducting wires in parallel.
The following will describe the oxide superconducting wire material (high temperature superconducting wire). One of possible preferable high-productivity methods of producing high-temperature superconductor elements is, for example, that of forming a film of oxide superconducting material on a flexible tape substrate. Production methods based on the vapor phase deposition method, such as laser ablation method, CVD method, etc., are now being developed. Oxide superconducting wires made by forming an oxide superconducting film on the tape substrate as described above have an exposed superconducting film on the outermost layer, and no stabilization treatment has been applied on the surface of the exposed side. As a result, when a relatively strong current is applied to such an oxide superconducting wire, the superconducting film transits locally from the superconducting state to the normal conducting state due to the local generation of heat, resulting in an unstable transmission of current.
For the purpose of solving the problems mentioned above, and providing an oxide superconductor having a high critical current value, capable of transmitting current with stability and whose stability does not deteriorate even after an extended period of storage and the method of producing the same, Japanese Patent Application Laid Open 7-37444 (pp. 2-7, FIG. 1) (hereafter Reference 2) discloses the following tape-shaped superconducting wire: “[A] superconducting wire comprises of an intermediate layer formed on a flexible tape substrate, an oxide superconducting film formed on the intermediate layer, and a gold or silver film (a metal normal conduction layer) 0.5 μm or more thick formed on the oxide superconducting film.” And example of embodiment described in Reference 2 reads as follows: “On ‘Hastelloy’ tape serving as the substrate, an yttria stabilized zirconia layer or magnesium oxide layer is formed as an intermediate layer. On top of this layer, Y—Ba—Cu—O oxide superconducting film is formed. And on this layer, a gold or silver coating film is formed.” However, when mass-produced tape-shaped superconducting wires like the ones described in References 2 are used in an AC device, the AC loss that develops in the superconducting wires will be, due to the form anisotropy of flat tapes, dominated by those in the perpendicular magnetic field acting in the perpendicular direction upon the flat surface of the tape, and thus the AC losses increase. In addition, there is a problem with regard to the transposition structure. To solve these problems, some of the inventors of the present application have disclosed the following superconducting wire materials and a superconducting coil based on the same materials in a related application PCT/JP2004/009965, corresponding to U.S. patent application Ser. No. 10/514,194, the disclosure of which is incorporated herein by reference.
FIGS. 6A, 6B, and 6C show a superconducting wire material disclosed in FIG. 1 of the international application mentioned above. Specifically, the international application has been contemplated for “providing a superconducting wire capable of suppressing AC loss and a low-loss superconducting coil made from this superconducting wire having a simple structure without transposition, capable of canceling interlinkage magnetic flux due to the perpendicular magnetic field to the wire, and capable of suppressing the circulating current within the wire due to the perpendicular magnetic field and making shunt current uniform so that the losses may be limited.” The international application, as shown in FIGS. 6A, 6B, and 6C further discloses the following: “[A] simple coil structure without transposition wherein a superconducting film formed on the substrate 31 is transformed into a tape to make a superconducting wire material, the superconducting film part constituting at least a superconducting layer 33 is slit to form slits 35 and to separate electrically the same into a plurality of superconducting film parts respectively having a rectangular section and arranged in parallel to form parallel conductors, in other words parallel conductors constituted by arranging a plurality of element conductors, and the superconducting coil constituted by winding the superconducting wire material has, in view of the structure or arrangement of the superconducting coil, a coil structure containing at least partially a part wherein the perpendicular interlinkage magnetic flux acting among various conductor elements 30 of the parallel conductors by the distribution of the magnetic field generated by the superconducting coils acts to cancel each other is provided.”
In FIGS. 6A, 6B, and 6C, the group number 30 represents a conductor element composed of split parts of a metal layer and a superconducting layer, and 32 represents an intermediate layer, 34 represents a metal layer, 35 represents a slit as splitting groove, and 36 represents an electric insulating material. The superconductor before splitting shown in FIG. 6A consists of, for example, Hastelloy tape for the substrate 31, on which the intermediate layer 32 is formed as an electric insulation layer, on which Y—Ba—Cu—O oxide superconducting film is formed as a superconducting layer 33, and on which, for example, a gold or silver coating layer is formed as a normal or non-superconducting conducting metal layer 34. Incidentally, as the intermediate layer 32 described above, a double-layered structure consisting of, for example, a cerium oxide (CeO2) layer formed on a gadolinium zirconium oxide (Gd2Zr2O7) layer is formed. The metal layer 34, however, need not be formed.
The superconducting conductor is, as shown in FIG. 6B, slit in the longitudinal direction of the superconducting conductor, and as shown in FIG. 6C epoxy resin, enamel, and other flexible electric insulation materials 36 are filled in the grooves formed by slitting and over the entire environment around the conductors to form parallel conductors. In applying the superconducting wires as described above to the superconducting coil, the superconducting wires consisting of the parallel conductors are, as shown in FIG. 6B, wound in the form of a cylindrical layer on the peripheral surface of a cylindrical bobbin made of an electrical insulation material not shown around the central axis of coil 14.
The superconducting wire material shown in FIGS. 6A, 6B, and 6C above functions as a multi-filament superconductor, enables to uniformize the sharing of current, and to reduce the magnetic field applied at right angles to the superconductor elements, to reduce AC losses by dividing the superconducting film part into a plurality and arranging them electrically in parallel.
In addition, the international application described above further discloses a preferable structure of superconducting coil to which the superconducting wire materials shown in FIGS. 6A, 6B, and 6C above are applied. Specifically, the international application states: “The superconducting coil constituted by winding the superconducting wire material has, in view of the structure or arrangement of the superconducting coil, a coil structure containing at least partially a part wherein the perpendicular interlinkage magnetic flux acting among various conductor elements of the parallel conductors by the distribution of the magnetic field generated by the superconducting coils acts to cancel each other is provided. This will provide a superconducting wire capable of suppressing AC loss and a low-loss superconducting coil made from this superconducting wire having a simple structure without transposition, capable of canceling interlinkage magnetic flux due to the perpendicular magnetic field to the wire, and capable of suppressing the circulating current within the wire due to the perpendicular magnetic field and making shunt current uniform so that the losses may be limited.” See the international application mentioned above for details.
The following will now describe the measures against over-current in the event of short-circuit of a transformer. When a transformer is short-circuited, strong short-circuit current flows in the coil and an excessive electromagnetic force works. In the case of a superconducting transformer, current density is higher than that of a normal conductive transformer. In other words, for a same current capacity, the superconducting transformer has a smaller conductor section. Therefore, when a same electromagnetic force works on the conductor, the superconducting transformer applies a larger stress to the conductor. In the case of an oxide superconducting transformer, the conductor, being an oxide, has a relatively low mechanical strength, and may not be able to withstand this electromagnetic force at the time of over-current.
The means for solving this problem is disclosed in Japanese Patent Application Laid Open 2001-244108 (hereafter Reference 3). The following is a citation of a summary contained in Reference 3: “On a superconducting coil constituted by winding a taped-shaped superconducting wire material along a spiral groove formed on the outer periphery of a cylindrical insulating bobbin, a metal tape wherein normal conductors such as copper, copper alloy, titanium, stainless steel and the like are used is lap wound on the outer periphery of the superconducting wire material mentioned above, the metal tape is bound by hardening the resin used, and then the metal tape is connected electrically in parallel with the superconducting wire material. This structure will enable to support the electromagnetic force in the radius direction applied to the superconducting wire material by the metal tape from the outer periphery in the event of a short-circuit, and to prevent possible burn-out of the coil due to a sharp rise in temperature by diverting a part of current to the metal tape when the superconducting wire material transformed into a normal conductor because of Joule generation of heat resulting from an over-current.”
The critical current of high-productivity tape-shaped superconducting wire materials such as those described in Reference 2 or the international application mentioned above is approximately 100 A in the self-magnetic field and at the liquid nitrogen temperature (77K). Under the superconducting coil state, the critical current falls down further due to the generation of the magnetic field, and the current usable for equipment falls down substantially from the critical current 100 A mentioned above. On the other hand, the required current capacity is varied according to the equipment used or usage. When a strong current is required as in the case of the low-voltage winding of a transformer for example, it is possible that the application described in Reference 2 or the international application mentioned above may be insufficient to cope with the situation.
Furthermore, at the time of starting excitation or in the event of an unexpected short-circuit for example, so-called measures against over-current may be required so that the AC equipment can withstand a current in excess of the rated current for a short period. On the tape-shaped superconductor elements described in Reference 2 or the international applications mentioned above, a metal layer consisting of gold or silver is formed as a stabilizing layer as described above. This metal layer is formed mainly for the purpose of improving superconductive performance. This metal layer, however, is generally 10 μm thick or less, making it too thin, and often insufficient to rely on as a safety measure against over-current.
Accordingly, there still remains a need to reduce AC losses, to increase the current capacity of coils, to prevent the burn-out of conductors due to over-current at the time of starting excitation or in the unexpected event of short-circuit by using parallel superconducting conductors and to provide a safe large-capacity superconducting coil. The present invention addresses this need.